1. Field of Invention
The field of the present invention relates in general to modems and more particularly digital modems.
2. Description of the Related Art
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. Subscriber lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud. With X-DSL significant increases in bandwidth have been made possible by utilizing frequencies higher than the voice band to deliver services such as: data, video, audio etc. Thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation.
Currently there are over ten X-DSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there may be more than one line code, or modulation protocol, e.g. discrete multi-tone (DMT) and carrier less AM/PM (CAP). DMT modulation involves establishing a communication channel with a plurality of sub-channels each with a center frequency a.k.a. carrier tone. The sub-channels are frequency division multiplexed across the available bandwidth. Each sub-channel uses quadrature phase amplitude modulation (QPAM) to modulate information. The center frequency, a.k.a. tone of each sub-channel serves as the carrier on which QPAM modulation of information is effected. The information modulated on a tone is identified in the frequency domain as a sub-symbol which defines a unique phase and amplitude relationship between the carrier tone and the information modulated on it. Each sub-symbol may be expressed as a complex number. Specific bits of information are converted to a corresponding complex number using a mapping table, which defines for all possible phase and amplitudes supported by the DMT protocol corresponding binary bits. Collectively all the sub-symbols modulated on each tone across a tone set are defined as a symbol, with the symbol rate defined by the corresponding X-DSL standard.
Typically initialization of an X-DSL modem utilizing a DMT modulation protocol, begins with several discrete phases of initialization including: handshake, training, channel analysis and showtime. The outcome of initialization is a contract between the modems on communication parameters which takes into account both modem capabilities and limitations in the quality of the communication medium linking the modems.
During the handshake phase each modem passes to the other basic identity information such as: inverse or discrete Fourier transform size and the initial length of the cyclic extension. This information is passed using a modulation protocol such as binary phase shift key (BPSK) with a single carrier. The use of BPSK allows information to be exchanged without using DMT modulation. The next and subsequent initialization phases utilize DMT modulation with various degrees of complexity up to the showtime phase at which the full transport capabilities of the modems and the subscriber line by which they are coupled have been trained.
The training phase involves setting gains, synchronizing clocks, synchronizing of symbol boundaries, and determination of weights associated with time or frequency domain equalization and echo cancellation. All of these are accomplished with a fixed set of data, a.k.a. pilot tones, modulated on selected sub-channels. The pilot tones and the manner in which they are modulated are known to both sending and receiving modems. The channel analysis phase follows the training phase.
During channel analysis each tone is modulated with varying amount of data in order to maximize the data carrying capability of the communication medium that links the modems. During this phase the modems pass parameters which allow each to establish a bit loading for each tone across a tone set. At the completion of the channel analysis and exchange the modems enter showtime state in which communication of voice, data, video, etc. across the communication medium begins.
The trend in X-DSL communication is toward integrating optical network units (ONU) and X-DSL modems to bridge the gap between the high speed backbone provided by the public switched telephone network (PSTN) and other long haul providers and the subscriber residence. The very high-speed digital subscriber line (V-DSL) offers short haul bandwidth approaching that achievable with ONU. This bandwidth is achieved by extending the upper limits of the frequency range at which the modem is required to operate. The combination of enhanced bandwidth, very low signal to noise ratios, and the bundling of subscriber lines results in initialization procedures for VDSL modems which are more complex than is the case for the lower bandwidth X-DSL standards such as ADSL.
VDSL modems require during the training phase of initialization an exchange of information other than pilot tones over the DMT link that they are setting up. VDSL draft trial standard document entitled “Very-high bit-rate Digital Subscriber Lines (VDSL) Metallic Interface, Part 3: Technical Specifications of a Multi-Carrier Modulation Transceiver” at Chapter 11, Link Activation and Deactivation pp. 36–58, T1E1.4/2000-013R2 promulgated by the Committee T1-Telecommunications, Working Group T1E1 (DSL Access) Vancouver, Canada, Aug. 14–18, 2000 sets forth such a standard. The draft standard calls for pilot tones interlaced with message tones to convey information between the modems which identify: bandwidth allocation, radio frequency interference (RFI) bands, transmit power spectral density (PSD) in the downstream and upstream directions, whether power back off (PBO) is performed using a maximum receive PSD or using an upstream PSD mask, the reference PSD, and the overall length of the window at the transmitter. The modems utilize this information to establish communication parameters for the training and subsequent phases of operation.
What is needed is a modem with improved capabilities for handling information transfer on a complex modulation protocol such as DMT during the training phase.